1. Field of the Invention
The present invention relates to a medical device and more specifically, to a anchor device which is adapted to facilitate the positioning of an ablation element at a location where a pulmonary vein extends from the left atrial wall.
2. Description of the Related Art
Cardiac arrhythmia's, particularly atrial fibrillation, are a pervasive problem in modern society. Although many individuals lead relatively normal lives despite persistent atrial fibrillation, the condition is associated with an increased risk of myocardial ischemia, especially during strenuous activity. Furthermore, persistent atrial fibrillation has been linked to congestive heart failure, stroke, and other thromboembolic events. Thus, atrial fibrillation is a major public health problem.
Normal cardiac rhythm is maintained by a cluster of pacemaker cells, known as the sinoatrial (“SA”) node, located within the wall of the right atrium. The SA node undergoes repetitive cycles of membrane depolarization and repolarization, thereby generating a continuous stream of electrical impulses, called “action potentials.” These action potentials orchestrate the regular contraction and relaxation of the cardiac muscle cells throughout the heart. Action potentials spread rapidly from cell to cell through both the right and left atria via gap junctions between the cardiac muscle cells. Atrial arrhythmia's result when electrical impulses originating from sites other than the SA node are conducted through the atrial cardiac tissue.
In most cases, atrial fibrillation results from perpetually wandering reentrant wavelets, which exhibit no consistent localized region(s) of aberrant conduction. Alternatively, atrial fibrillation may be focal in nature, resulting from rapid and repetitive changes in membrane potential originating from isolated centers, or foci, within the atrial cardiac muscle tissue. These foci exhibit consistent centrifugal patterns of electrical activation, and may act as either a trigger of atrial fibrillatory paroxysmal or may even sustain the fibrillation. Recent studies have suggested that focal arrhythmia's often originate from a tissue region along the pulmonary veins of the left atrium, and even more particularly in the superior pulmonary veins.
Several surgical approaches have been developed for the treatment of atrial fibrillation. For example, Cox, J L et al. disclose the “maze” procedure, in “The Surgical Treatment Of Atrial Fibrillation. I. Summary”, Thoracic and Cardiovascular Surgery 101(3):402-405 (1991) and “The Surgical Treatment Of Atrial Fibrillation. IV. Surgical Technique”, Thoracic and Cardiovascular Surgery 101(4):584-592 (1991). In general, the maze procedure is designed to relieve atrial arrhythmia by restoring effective SA node control through a prescribed pattern of incisions about the cardiac tissue wall. Although early clinical studies on the maze procedure included surgical incisions in both the right and left atrial chambers, more recent reports suggest that the maze procedure may be effective when performed only in the left atrium (see for example Sueda et al., “Simple Left Atrial Procedure For Chronic Atrial Fibrillation Associated With Mitral Valve Disease” (1996)).
The left atrial maze procedure involves forming vertical incisions from the two superior pulmonary veins and terminating in the region of the mitral valve annulus, traversing the inferior pulmonary veins en route. An additional horizontal incision connects the superior ends of the two vertical incisions. Thus, the atrial wall region bordered by the pulmonary vein ostia is isolated from the other atrial tissue. In this process, the mechanical sectioning of atrial tissue eliminates the atrial arrhythmia by blocking conduction of the aberrant action potentials.
The moderate success observed with the maze procedure and other surgical segmentation procedures have validated the principle that mechanically isolating cardiac tissue may successfully prevent atrial arrhythmia's, particularly atrial fibrillation, resulting from either perpetually wandering reentrant wavelets or focal regions of aberrant conduction. Unfortunately, the highly invasive nature of such procedures may be prohibitive in many cases. Consequently, less invasive catheter-based approaches to treat atrial fibrillation through cardiac tissue ablation have been developed.
These less invasive catheter-based therapies generally involve introducing a catheter within a cardiac chamber, such as in a percutaneous translumenal procedure, wherein an energy sink on the catheter's distal end portion is positioned at or adjacent to the aberrant conductive tissue. Upon application of energy, the targeted tissue is ablated and rendered non-conductive.
The catheter-based methods can be subdivided into two related categories, based on the etiology of the atrial arrhythmia. First, focal arrhythmias have proven amenable to localized ablation techniques, which target the foci of aberrant electrical activity. Accordingly, devices and techniques have been disclosed which use end-electrode catheter designs for ablating focal arrhythmia's centered in the pulmonary veins, using a point source of energy to ablate the locus of abnormal electrical activity. Such procedures typically employ incremental application of electrical energy to the tissue to form focal lesions.
The second category of catheter-based ablation methods is designed for treatment of the more common forms of atrial fibrillation, resulting from perpetually wandering reentrant wavelets. Such arrhythmias are generally not amenable to localized ablation techniques, because the excitation waves may circumnavigate a focal lesion. Thus, the second class of catheter-based approaches have generally attempted to mimic the earlier surgical segmentation techniques, such as the maze procedure, wherein continuous linear lesions are required to completely segment the atrial tissue so as to block conduction of the reentrant wave fronts.
For the purpose of comparison, ablation catheter devices and related methods have also been disclosed for the treatment of ventricular or supraventricular tachycardias, such as disclosed by Lesh, M D in “Interventional Electrophysiology—State Of The Art, 1993” American Heart Journal, 126:686-698 (1993) and U.S. Pat. No. 5,231,995 to Desai.
An example of an ablation method targeting focal arrhythmia's originating from a pulmonary vein is disclosed by Haissaguerre et al. in “Right And Left Atrial Radiofrequency Catheter Therapy Of Paroxysmal Atrial Fibrillation” in J. Cardiovasc. Electrophys. 7(12):1132-1144 (1996). Haissaguerre et al. describe radiofrequency catheter ablation of drug-refractory paroxysmal atrial fibrillation using linear atrial lesions complemented by focal ablation targeted at arrhythmogenic foci in a screened patient population. The site of the arrhythmogenic foci was generally located just inside the superior pulmonary vein, and was ablated using a standard 4 mm tip single ablation electrode.
Another ablation method directed at paroxysmal arrhythmia's arising from a focal source is disclosed by Jais et al. “A Focal Source Of Atrial Fibrillation Treated By Discrete Radiofrequency Ablation” Circulation 95:572-576 (1997). At the site of arrhythmogenic tissue, in both right and left atria, several pulses of a discrete source of radiofrequency energy were applied in order to eliminate the fibrillatory process.
Application of catheter-based ablation techniques for treatment of reentrant wavelet arrhythmia's demanded development of methods and devices for generating continuous linear lesions, like those employed in the maze procedure. Initially, conventional ablation tip electrodes were adapted for use in “drag burn” procedures to form linear lesions. During the “drag” procedure, as energy was being applied, the catheter tip was drawn across the tissue along a predetermined pathway within the heart. Alternatively, sequentially positioning the distal tip electrode, applying a pulse of energy, and then re-positioning the electrode along a predetermined linear pathway also made lines of ablation.
Subsequently, conventional catheters were modified to include multiple electrode arrangements. Such catheters typically contained a plurality of ring electrodes circling the catheter at various distances extending proximally from the distal tip of the catheter. More detailed examples of such catheter-based tissue ablation assemblies have been disclosed in U.S. Pat. No. 5,676,662 to Fleischhacker et al.; U.S. Pat. No. 5,688,267 to Panescu et al.; and U.S. Pat. No. 5,693,078 to Desai et al.
Examples of catheter-based cardiac chamber segmentation procedures, particularly in the treatment of Wolff-Parkinson-White syndrome, are disclosed by Avitall et al. “Physics And Engineering Of Transcatheter Tissue Ablation” J. Am. College of Cardiology, 22(3):921-932 (1993) and Haissaguerre et al. “Right And Left Atrial Radiofrequency Catheter Therapy Of Paroxysmal Atrial Fibrillation” J. Cardiovasc. Electrophys. 7(12):1132-1144 (1996).
Further more detailed examples of transcatheter-based tissue ablation assemblies and methods are described in the following references: U.S. Pat. No. 5,575,810 to Swanson et al.; PCT Published Application WO 96/10961 to Fleischman et al.; U.S. Pat. No. 5,702,438 to Avitall; U.S. Pat. No. 5,687,723 to Avitall; U.S. Pat. No. 5,487,385 to Avitall; and PCT Published Application WO 97/37607 to Schaer.
While the disclosures above describe feasible catheter designs for imparting linear ablation tracks, as a practical matter, most of these catheter assemblies have been difficult to position and maintain placement and contact pressure long enough and in a sufficiently precise manner in the beating heart to successfully form segmented linear lesions along a chamber wall. Indeed, many of the aforementioned methods have generally failed to produce closed transmural lesions, thus leaving the opportunity for the reentrant circuits to reappear in the gaps remaining between point or drag ablations. In addition, minimal means have been disclosed in these embodiments for steering the catheters to anatomic sites of interest such as the pulmonary veins. Subsequently, a number of attempts to solve the problems encountered with precise positioning, maintenance of contact pressure, and catheter steering have been described. These include primarily the use of (1) preshaped ablating configurations, (2) deflectable catheter assemblies, and (3) transcatheter ablation assemblies.
None of the catheter-based ablation assemblies have included a balloon anchor wire for positioning and anchoring one end of an elongated ablation member within the ostium of a pulmonary vein. Nor does the prior art disclose a method for securing the ablation member between a first and second anchor, thereby maintaining a desired linear position in contact with the atrial wall and facilitating the formation of a linear ablation track along the length of tissue between the anchors.